In the manufacture of heavy duty gears, it is generally necessary to harden the tooth and root surfaces by a heat treating process to improve their wear resistance. For best results, the depth of hardening (case) should be limited, since through hardening of the teeth produces internal stresses that may result in cracking. Gears so treated are said to be contour hardened since the hardened region follows the contour of the gear.
To harden gears by induction, a work coil (inductor) is brought in close proximity to the gear and energized with alternating current. The coil is energized at a predetermined frequency and power level for a predetermined time to achieve the desired heating, whereafter the gear is quenched to produce the desired hardness. The frequency, power level and time parameters are interrelated and chosen for a given application in relation to the size of the gear and the required case depth. Due to a phenomenon known as skin effect, the depth of heating varies inversely with the frequency at which the coil is energized. Time of heating relates to the phenomenon of conduction within the workpiece. Due to the phenomenon of conduction, surface heat rapidly progresses to the core of the gear, thereby increasing the depth of heating with increasing time of heating. The coil power density is related to the coil size and the power level applied to the coil, and directly affects the amount of localized heating.
Broadly, there are two different techniques for hardening gears by induction: scanning and single shot. In the scanning technique, the coil spans only a portion of the gear, and either the coil or gear is moved relative to the other so that the coil is passed along the width of one or more gear teeth. After quenching the heated teeth, the gear or coil is then indexed, and the process repeated until the entire gear has been hardened. In the single shot technique, all of the gear teeth are heated concurrently by a coil which surrounds the gear. After the heating is completed, the teeth are concurrently quenched. Of the two techniques, single shot has the advantages of being significantly faster to perform, and less likely to result in distortion and overheating of the gear edges.
Presently known single shot induction hardening techniques require two frequencies of coil energization and comprise four steps: preheat, dwell, final heat, and quench. In the preheat step, the gear is positioned relative to a first inductor, which inductor is energized for a predetermined interval at a relatively low frequency and power level to heat the teeth and roots to a predetermined temperature. In the dwell step, the heating is interrupted for a predetermined interval during which the the tips of the gear teeth are allowed to cool in preparation for final heating. Depending on the pitch of the gear teeth, the dwell step may be minimized or eliminated entirely. In the final heat step, the gear is positioned relative to a second inductor, which inductor is energized for a predetermined interval at a relatively high frequency and power level to produce a very localized surface heating of the teeth and roots. In the fourth step, the gear is quenched. In any given application, the preheat and dwell intervals are selected to heat the roots and tips of the gear to desired temperatures, and the final heat interval is selected such that both the tips and roots are at uniform higher temperature at the time of quenching. In a typical application, the preheat step is carried out with a solid state power supply operating at a frequency of 1-10 kHz and a power level of about 30 kW for a period of 30-35 seconds, while the final heat step is carried out with a vacuum tube oscillator power supply operating at a radio frequency (rf) such as 450 kHz and a power level of 350 kW for a period of 0.4-2.0 seconds, depending on the gear pitch. Such known single shot technique has been successfully used to contour harden gears, but has the disadvantage of requiring different power supplies and work stations for carrying out the preheat and final heat steps.